Laser range finders have been known for a considerable time. These devices are used, for example, by surveyors to calculate the distance from a point of observation to an object such as a geological formation in the field of view. Generally, a laser range finder operates by projecting a pulse of laser light at an object. The laser light illuminates the object, and a portion of the laser light is reflected back toward the laser range finder device. The reflected laser light is detected, and the time interval required for the laser light pulse to travel to and from the object is measured. From this time interval measurement and the known speed of light, the distance between the laser range finder and the object is calculated.
Such a conventional laser range finder includes a laser capable of producing laser light pulses of high peak power and very short duration (i.e., less than 50 ns duration). The detector for the reflected laser light may include a high speed photodetector (such as an InGaAs avalanche photodiode), which is coupled to a high gain, high speed amplifier. A high speed digital counter is used as a timer to determine the time interval required for the laser light to travel to the object and for laser light reflecting off of the object to travel back to the device. From this time interval information an internal electronic calculator determines the range to the object, and this range is presented to the user of the device, usually on a visual display screen.
These conventional laser range finders have a disadvantage of a considerable cost and complexity. The laser pulses must be of considerable intensity as well, which requires a high power laser. The conventional laser range finders are subject to optical and electrical problems, such as vulnerability to electromagnetic interference, damage to electrical components and damage to optical components. Reliability of the devices is also adversely impacted by their complexity.
On the other hand, conventional night vision devices of the image intensification type (i.e., light amplification) type have also been known for a considerable time. Generally, these night vision devices include an objective lens which focuses invisible infrared light from the night time scene onto the transparent light-receiving face of an image intensifier tube. At its opposite image-face, the image intensifier tube provides an image in visible yellow-green phosphorescent light, which is then presented to a user of the device via an eye piece lens.
Even on a night which is too dark for diurnal vision, invisible infrared light is richly provided by the stars. Human vision can not utilize this infrared light from the stars because the so-called near-infrared portion of the spectrum is invisible for humans. A night vision device of the light amplification type can provide a visible image replicating the night time scene.
A contemporary night vision device will generally use an image intensifier tube with a photocathode behind the light-receiving face of the tube. The photocathode is responsive to photons of infrared light to liberate photoelectrons. These photoelectrons are moved by a prevailing electrostatic field to a microchannel plate (MCP) having a great multitude of dynodes, or microchannels with an interior surface substantially defined by a material having a high coefficient of secondary electron emissivity. The photoelectrons entering the microchannels cause a cascade of secondary emission electrons to move along the microchannels so that a spatial output pattern of electrons which replicates an input pattern, and at a considerably higher electron density than the input pattern results. This pattern of electrons is moved from the microchannel plate to a phosphorescent screen to produce a visible image. A power supply for the image intensifier tube provides the electrostatic field potentials referred to above, and also provides a field and current flow to the microchannel plate.
Conventional night vision devices (i.e., since the 1970's and to the present day) provide automatic brightness control (ABC), and bright-source protection (BSP). The former function maintains the brightness of the image provided to the user substantially constant despite changes in the brightness (in infrared and the near-infrared portion of the spectrum) of the scene being viewed. BSP prevents the image intensifier tube from being damaged by an excessively high current level in the event that a bright source, such as a flare or fire, comes into the field of view.
The ABC function is conventionally accomplished by providing a regulator circuit monitoring the output current from the phosphorescent screen. When this current exceeds a certain threshold, the field voltage level across the opposite faces of the microchannel plate is decreased to reduce its gain. In other words, for each photoelectron entering a channel of the microchannel plate, the number of secondary-emission electrons exiting this microchannel will be a function of the applied voltage differential across the microchannel plate (as well as the so-called "strip current" of this plate) within a certain range for the operation of the microchannel plate. Reducing the applied voltage differential across a microchannel plate decreases both the number of secondary-emission electrons produced and the strip current of the microchannel plate.
Unfortunately, this reduction in microchannel plate voltage also has the effect of reducing the resolution of the image intensifier tube. That is, the gain versus voltage function of the image intensifier tube at lowered voltages results in a matrix pattern from the microchannel plate appearing in the image. This matrix pattern is sometimes referred to as fixed-pattern noise in the image. As a result, in bright-field conditions with the ABC feature of the conventional night vision device operating the conventional night vision device may drastically lose resolution so that the user of the device is no longer able to discern details of the viewed scene which would be discernable were they viewed under darker field conditions in which ABC function were not applying.
A bright-source protection is provided in conventional night vision devices by decreasing the electrostatic field voltage provided to the photocathode. A BSP function results because the high impedance of the photocathode in combination with a circuit element of high resistance value in series with the photocathode creates a greater voltage drop under the higher current conditions caused by a large number of photons incident on the photocathode when a bright source is present in the viewed field (i.e., with a resulting high number of photoelectrons being provided by the photocathode). The photoelectrons provided by the photocathode represent a current flow increasing in magnitude with increasing light levels in the viewed field, such that the impedance of the photocathode causes an inherent decrease in the voltage level effective at the photocathode to move these electrons to the microchannel plate.
This conventional method of BSP also has the disadvantage of decreasing resolution for the image intensifier tube. The reduced electrostatic field between the photocathode and the microchannel plate input when the BSP function is operational causes a reduced resolution for the tube. That is, photoelectrons liberated within the photocathode are not moved to the microchannel plate as effectively when the field voltage for the photocathode is reduced, and may not reach the microchannel plate at all. This is the case because photoelectrons within the photocathode must overcome a surface potential barrier in order to escape the photocathode and move to the microchannel plate. In order to be liberated into free space and be moved by the prevailing electrostatic field to the input of the microchannel plate, photoelectrons must have a certain effective energy level. As the voltage applied to the photocathode decreases, considered statistically, some photoelectrons will not be able to overcome this surface potential barrier and will not be liberated into free space. The image information represented by these photoelectrons trapped within the photocathode will be lost from the image provided to the user of the night vision device.
Conventional night vision devices which are usable to sight a weapon are found in U.S. Pat. Nos. 5,084,780; and 5,035,472. Neither of these patents is believed to suggest or disclose a night vision device which is combined with a laser range finder using the image intensifier tube of the night vision device as a detector for laser light in the laser range finder.